The present invention is directed generally to motor arrangements for driving display hands, rings and other indicators for small (i.e. wearable) electronic devices, and in particular, to a micro-electromechanical systems (MEMS) driving arrangement for an electronic device, and in a particularly desirable embodiment, for driving such hands, rings and/or other indicators in a timepiece, such as a wristworn device. However, it will be understood from the disclosure herein that the invention is not so limiting.
Micro-electromechanical systems (MEMS) that are used as unidirectional and bidirectional drive units are known in the art. For example, one known MEMS drive unit is a bi-directional arrangement that uses at least four individual MEMS actuator portions, which are forming two pairs of individual MEMS actuators that mesh at least at two individual areas with the driven wheel. For each rotating direction, the drive unit requires one pair of MEMS actuator portions, where alternately one of the pairs is driving the wheel while the other one is pulled out of the contact with the wheel. The other pair of actuator portions is needed for rotating the wheel in the opposite driving direction. While driving the wheel in either direction, the actuator portions of the other pair (i.e. for the other direction) must also be pulled out of meshing with the wheel. Each of the four individual actuator portions is an electrostatic activation area, where one area within a pair of MEMS actuators is for generating a tangential driving force or torque to drive the wheel and the other area within the pair is for generating a radial force to pull the drive unit out of meshing with the wheel. An arrangement of the foregoing type thus requires four (4) electrostatic portions.
Another wheel driving actuator design is described in U.S. Pat. No. 7,592,737. In this design, a MEMS device is provided which includes a driven element having a series of teeth. The MEMS device includes a driving element operable to engage the driven element when the driving element is in a meshed position with the series of teeth. A pair of individual driving actuators of the MEMS device moves the driving element with a hysteresis-type motion in and out of meshing with the driven element. Another individual MEMS actuator is moving radially and must be engaged with the wheel to prevent undesired rotation, while the other pair of driving actuators is pulled out of meshing.
It is believed that the existing state of the art has deficiencies, and further believed that advances to the state of the art are both desirable and achievable. For example, in each of the foregoing designs, there are more actuators needed than would be desired. For example, in the first mentioned example above, four actuator portions are needed. In the latter example of the '737 patent, at least three MEMS actuators are needed. In addition, the foregoing arrangements are more complicated than desired for minimizing or preventing undesired motion of the driven wheel.
Accordingly, it is desired to provide a driving arrangement where all of the needed objectives and advantages can be achieved by the use of two MEMS actuators. Moreover, because of the improved design of the driving actuation assembly of the present invention, the preferred embodiments can provide for constant engagement of the driven wheel, thus preventing undesired motion thereof in addition, the present invention's design further leads to a reduced number of needed control signals than heretofore appreciated to achieve all of the needed functionality. Other objectives and features that address deficiencies in the prior art are also provided as disclosed herein.